In closed environments such as submarines, spacecraft, and spacesuits, atmospheric carbon dioxide partial pressures increase in concentration due to respiration. Increased carbon dioxide partial pressures can be a health hazard since carbon dioxide concentrations exceeding about 7.6 millimeters of mercury (mmHg) (partial pressure of 1.0%) are not safe for breathing for extended periods of time. As a result, in order to maintain a safe, habitable environment, it is necessary to remove carbon dioxide from these closed environments and thereby attain an acceptable carbon dioxide concentration below about 3.8 mmHg (partial pressure of 0.5%).
Carbon dioxide can be removed from these environments by passing the carbon dioxide containing atmosphere through a reactor containing a selective carbon dioxide sorbent. In the reactor, the carbon dioxide is absorbed by the sorbent and therefore removed from the closed environment. Typical carbon dioxide sorbents include regenerable and non-regenerable molecular sieves, metal oxides, alkali metal hydroxides, alkali metal carbonates, and others.
A particularly useful regenerable carbon dioxide sorbent capable of high carbon dioxide loadings, up to approximately 20 lbs/ft.sup.3, is a metal oxide-alkali metal carbonate sorbent, such as a silver oxide-cesium carbonate sorbent. (see Application U.S. Ser. No. 07/544,716) The postulated reaction mechanism for carbon dioxide removal using a silver oxide-cesium carbonate sorbent is: EQU Ag.sub.2 O+H.sub.2 O.revreaction.2Ag.sup.+ +20H.sup.- ( 1) EQU Cs.sub.2 CO.sub.3 +H.sub.2 O.revreaction.2Cs.sup.+ +HCO.sub.3.sup.- +OH.sup.- ( 2) EQU CO.sub.2 +OH.sup.- .revreaction.HCO.sub.3.sup.- ( 3) EQU HCO.sub.3.sup.- +OH.sup.- .revreaction.H.sub.2 O +CO.sub.3.sup.= ( 4) EQU 2Ag.sup.+ +CO.sub.3.sup.= .revreaction.Ag.sub.2 CO.sub.3 ( 5)
Basically, the silver oxide (Ag.sub.2 O) reacts with water (H.sub.2 O) to form silver ions (Ag.sup.+) and hydroxide ions (OH.sup.-) (Equation 1) while the cesium carbonate (CsCO.sub.3) reacts with water to form hydroxide ions, cesium ions (Cs.sup.+), and bicarbonate ions (HCO.sub.3.sup.-) (Equation 2). Carbon dioxide (CO.sub.2) then reacts with the hydroxide ions to form bicarbonate ions (Equation 3) which react with additional hydroxide ions to form carbonate ions and water (Equation 4). Finally, the carbonate ions react with the silver ions to form silver carbonate (Ag.sub.2 CO.sub.3) (Equation 5), thereby regenerating the cesium carbonate for additional carbon dioxide removal.
As is evident from the above equations, the absorption of carbon dioxide by a metal oxide-alkali metal carbonate sorbent requires the presence of hydroxide ions, and therefore water. Comparison of the rates of reaction for the absorption reaction, Equations 1-5, reveals that Equation 3, the reaction between the carbon dioxide and the hydroxide ions, is the slowest reaction and therefore rate determining reaction. As a result, the ability of the sorbent to absorb carbon dioxide is dependent upon the relative humidity of the atmosphere. At low relative humidities, below about 25%, a substantial decrease in carbon dioxide sorption rate is observed due to the reduced amount of hydroxide ions available to react with the carbon dioxide (see Equation 3).
Since the removal of carbon dioxide with the aforementioned sorbent is an exothermic reaction, the sorbent temperature increases as carbon dioxide is absorbed. As a result of the increased sorbent temperature, the relative humidity above the sorbent decreases causing water to evaporate from the sorbent reducing the amount of moisture available for reaction with carbon dioxide, and in turn, decreases the carbon dioxide sorption rate. Consequently, low relative humidity conditions require an active sorbent cooling system and/or additional sorbent in order to maintain the carbon dioxide sorption rate. However, volume, weight, and energy restrictions limit the use of these alternatives in most closed environment systems.
What is needed in the art is a carbon dioxide sorbent capable of maintaining carbon dioxide sorption rates associated with high relative humidities at relative humidities below about 25%.